Clear Books provides a unique email address for each account which you can use to send an email with multiple files. These files can then be attached to the appropriate transactions from within Clear Books.
The file types supported are png, gif, jpg, jpeg, tif, tiff, pdf and csv.
Navigate to Settings > Organisation > Details.
Click on the API tab. You'll see the email address which can be used to upload multiple attachments into your account.
If you create or edit an invoice/quote/bill/PO/Expense and you click on the manage attachments button or link, all the files you sent should be listed waiting to be attached to the specific transaction.